This invention relates to internal combustion engines, and more particularly to a centrifugally responsive mechanical compression release.
Compression release mechanisms are common in pull-start engines to make the engines easier to start. In a normal pull-start engine, the operator pulls a rope which moves the engine through one or more cycles. During the compression stroke of the engine cycle, the operator must exert enough force to compress the air in the combustion chamber, and the additional force from compressing the air makes it more difficult to start the engine. In a pull-start engine with a compression release mechanism, pressure in the combustion chamber is slightly released during the compression stroke to reduce the resistive force on the rope. This makes the engine easier to start because the operator does not have to pull the rope as hard. Typically, a compression release mechanism slightly unseats the exhaust valve to vent the combustion chamber during starting while the engine is revolving at cranking speeds. The mechanism then typically disengages when the engine reaches normal operating speeds.
Some compression release mechanisms use centrifugal forces to disengage themselves from the cam follower. These designs generally have a cam member and a flyweight. When the cam shaft rotation speed reaches a certain point, the flyweight moves away from the cam shaft, which positions the cam member out of contact with the cam follower. Some previous saddle-type compression release designs had pivot points on the cam shaft that required machining or drilling of the cam shaft. Modifying and machining a cam shaft is difficult because of its hardness and curved surface. The flyweights of some saddle-type designs also required apertures in the cam gear for clearance.
Other compression release mechanisms involve complex shapes that are difficult to manufacture and assemble. Complex designs usually require additional manufacturing steps which increase the cost of the part. Also, a complex part usually takes longer to assemble and is more likely to be assembled improperly.
The present invention includes a cam with a cam lobe and base radius. The cam is preferably slip fit over a cam shaft. A compression release member is preferably located adjacent the base radius and retained by a retainer, although the compression release member could be placed in other locations. In one embodiment, the compression release member is disposed in a slot on the base radius. The compression release member preferably comprises a first portion, a second portion, and a bridging portion that interconnects the first and second portions. Preferably, the first portion has an arc-shaped auxiliary cam surface that engages a cam follower, and the second portion functions as a flyweight. The pivot pin is preferably disposed within the curvature of the bridging portion and retains the compression release member in the slot. The bridging portion contacts the back surface of the slot, which absorbs the forces the cam follower applies on the compression release member. In the preferred embodiment, the compression release member may be symmetrical about a line through the bridging portion, but by no means is the invention limited to this embodiment. A symmetrical design provides additional benefits, but is not necessary to practice this invention.
In operation, the cam follower contacts the cam lobe as the cam shaft rotates. The compression release member is located in a slot along the base radius. At low speeds, the auxiliary cam surface engages the cam follower and slightly lifts the cam follower from the cam. Once the engine reaches higher running speeds, centrifugal forces pivot the compression release member out of contact with the cam follower.
The present invention achieves many advantages over previous compression release mechanisms. Biasing springs are not needed when the invention is incorporated into vertical shaft engines. The costly process of machining the cam shaft is no longer necessary because the compression release member is preferably integrated into the cam, which can be slip fit over the cam shaft. This arrangement can be readily integrated into an engine utilizing a cam lever and direct lever overhead valve system.
The back surface of the slot bears the forces the cam follower applies upon the compression release member. This substantially flat back surface is capable of supporting a relatively large amount of force and minimizes the forces applied on the pivot pin. The auxiliary cam surface is curved so there are no corners to cut into the cam follower. The cam follower is also preferably curved, and this smooth transition of the cam follower from the base radius to the compression release member extends the life of the parts.
In the preferred embodiment applied to a 5 hp engine, the compression release member is approximately 0.375 inches wide. This width dimension is wider than most previous compression release mechanisms and allows the forces transferred to the back surface to be distributed along a larger surface area. One skilled in the art will realize the invention does not require this large of a width dimension, and the size of the compression release member ultimately depends on the size of the cam lobe and the engine. The invention is by no means limited to this dimension, which merely provides an additional benefit of the preferred embodiment.
Additional advantages of this invention are derived from its efficient design. The compression release member may be easily stamped, or cut from a metal coil and bent into the proper shape. As previously mentioned, the compression release member may be symmetrical about a line through the bridging portion. While not necessary, the symmetrical design provides benefits during assembly of the invention. Since both the first and second portions are the substantially the same in this embodiment, either arced surface may be the auxiliary cam surface; the compression release member cannot be placed in the slot upside-down. This feature saves time during the assembly process, eliminates many mis-assembled parts, and reduces costs.